The present invention relates to a traffic control method and system for an ATM switching apparatus and, more particularly, to a traffic control method and system for an ATM switching apparatus, which control a traffic on the basis of a predetermined transmission capacity in an ATM switching apparatus for performing switching connection of fixed-length packets called cells.
In general, in an ATM switching apparatus for performing switching connection of fixed-length packets called cells on the basis of the Asynchronous Transfer Mode (ATM), the flow rate of cells, i.e., the traffic, in each virtual communication channel is monitored to control the traffic within an assigned transmission capacity.
FIG. 5 shows a conventional ATM switching apparatus traffic control system like the one disclosed in Japanese Patent Laid-Open No. 3-218143. Referring to FIG. 5, reference numeral 1 denotes a queuing buffer; 2, a cell held in the queuing buffer 1; 3, an inflow cell; 4, an outflow cell; 5, a residual amount monitoring section for monitoring the amount of cells 2 held in the queuing buffer 1, i.e., the residual amount of cells 2, on the basis of a predetermined residue threshold; 6, a residual amount monitor output from the residual amount monitoring section 5; and 7, an inflow cell restraint determining section for outputting an inflow cell restraint output 8 for designating an inflow restraint of the inflow cells 3 on the basis of the residual amount monitor output 6.
A conventional traffic control system will be described next with reference to FIGS. 6A to 6C. FIG. 6A shows the residual amount of cells in the queuing buffer 1. FIGS. 6B and 6C respectively show the residual amount monitor output 6 and the inflow cell restraint output 8. The residual amount monitoring section 5 monitors the amount of cells 2 held in the queuing buffer 1, i.e., a residual cell amount 41 (FIG. 6A) at intervals of unit times T, i.e., time t.sub.n-2, time t.sub.n-1, . . . Before time t.sub.n the residual cell amount 41 is below a residue threshold 42. The residual amount monitor output 6 is set at "L" level, as shown in FIG. 6B, and the inflow cell restraint output 8 from the inflow cell restraint determining section 7 is also set at "L" level, as shown in FIG. 6C. No restraint is therefore imposed on inflow cells.
Assume that the residual cell amount 41 exceeds the residue threshold 42 after time t.sub.n. In this case, the residual amount monitoring section 5 recognizes this state at time t.sub.n+1 and sets the residual amount monitor output 6 at "H" level (FIG. 6B). When the inflow cell restraint determining section 7 sets the inflow cell restraint output 8 at "H" level in response to this operation without any delay (FIG. 6C), a restraint on the inflow cells 3 to the queuing buffer 1 is started at time t.sub.n+1. If the inflow of cells is completely stopped in performing this restraint, the residual cell amount 41 of cells 2 in the queuing buffer 1 decreases in accordance with the amount of outflow cells 4. Thereafter, the residual amount monitoring section 5 monitors at time t.sub.n +3 that the residual cell amount 41 becomes smaller than the residue threshold 42. As a result, both the residual amount monitor output 6 and the inflow cell restraint output 8 are set at "L" level to resume the inflow of cells.
In this conventional traffic control system for an ATM switching apparatus, when the residual cell amount 41 of the queuing buffer 1 exceeds the predetermined residue threshold 42, the inflow cells 3 are restrained. The restraint on the inflow cells 3 is delayed because of the offset between the residual amount monitor timing and the traffic variation. As a result, a large amount of loss cells 43 are generated. With an increase in transmission capacity, in particular, the amount of loss cells increases.